The invention relates to improvements in sutures and suturing techniques, and more particularly to materials and devices for making high-strength fused suture loops during surgical procedures.
In surgical procedures, a monofilamentous suture is typically used to stitch or secure the edges of tissue together to maintain them in proximity until healing is substantially completed. The suture is generally directed through the portions of the tissue to be joined and formed into a single loop or stitch, which is then knotted or otherwise secured in order to maintain the wound edges in the appropriate relationship to each other for healing to occur. In this manner a series of stitches of substantially uniform tension can be made in tissue. Because the stitches are individual and separate, the removal of one stitch does not require removal of them all or cause the remaining stitches to loosen. However, each individual stitch requires an individual knot or some other stitch-closing device for securing the stitch around the wound.
It is sometimes necessary or desirable to close a wound site with sutures without having to form knots or incorporate loop-closing devices in the sutures, such as, for example, in surgical repair of delicate organs or tissues, where the repair site is relatively small or restricted. A fused suture loop must provide the appropriate tension on the wound edges and the appropriate strength to maintain the wound edges in sufficient proximity for a sufficient time to allow healing to occur.
Polymer sutures are particularly amenable to various fusing or joining processes, such as, for example, welding, whereby sections of the sutures can be fused together upon application of sufficient heat to the sections to cause partial melting and fusion of the sections. Because the direct application of heat to sutures in situ may produce undesirable heating of the surrounding tissue, it is preferred to apply non-thermal energy to the suture material in situ to induce localized heating of the suture material in the areas or sections to be fused. In particular, ultrasonic energy may be effectively applied to sections of suture materials to induce frictional heating of the sections in order to fuse or weld them together.
While sutures typically fail under tensile loads applied along the principal axis of the suture, suture welds often fail in shear, i.e., in the plane of the fused region between the overlapped segments of suture material. It is desirable to have the failure strength of the suture joint be at least as great as the failure strength of the suture material away from the joint.
U.S. Pat. No. 5,417,700 to Egan and U.S. Pat. No. 3,515,848 to Winston et al. disclose apparatus and methods for ultrasonic welding of sutures. The Winston et al. patent discloses, for example, the application of mechanical energy to a segment of material to be joined in either of two different directions. For joining plastic suture materials, mechanical energy is applied in a direction substantially parallel to the axis of the segments to be joined. For joining metallic suture materials, mechanical energy is applied in a direction substantially normal to this axis. The Winston et al. patent further discloses the use of a spherical welding tip for use in joining metallic suture materials.
Although ultrasonic welding of sutures is known, it has heretofore been difficult or impossible to control the suture welding process in order to produce suture welds of sufficient strength and reliability to replace, or enhance the strength of, suture knots or other loop closure devices.
It is therefore an object of the present invention to overcome the disadvantages inherent in prior art suture loop joints and joining processes.
The present invention provides a fused loop of an elongated material, such as a polymeric or monofilamentous suture material, which has a strength in the joint region which is at least equal to, if not greater than, the strength of the parent material. The present invention also provides means for controlling the size and shape of the fused portion of the joint region in order to maximize joint strength.
According to one aspect of the invention, there is provided a fused loop of an elongated material comprising one or more segments of the material which extends along a principal axis. Portions of the segments are joined together to form a loop at a joint region which extends between first and second ends. The joint region includes a first portion of elongated material extending from the first end, a second portion of elongated material extending from the second end, and a fused portion or layer between and joining the first and second portions at points between the first and second ends of the joint region. The fused portion preferably comprises a relatively thin layer of fused material from the first and second portions.
The term xe2x80x9cfusedxe2x80x9d, as used herein, refers to material which has been heated to a plastic or fluid state and subsequently allowed to cool, so that the relatively highly-oriented molecular structure of the parent material is transformed into a relatively randomly-oriented molecular structure characterizing the fused portion of the joint region. The term xe2x80x9cshear areaxe2x80x9d, as used herein, refers to the area of the fused portion between and substantially parallel to the segments of material joined in the joint region. In contrast, the cross-sectional area of the segments or the fused portion refers to the area in a plane substantially transverse to the principal axis of the segments.
The elongated material in the first and second portions of the joint region is characterized by a relatively high degree of molecular orientation in the direction of the principal axis of the material, and thus relatively high strength in the direction of the principal axis. The fused material in the fused portion of the joint region is characterized by a relatively random molecular orientation, and thus relatively low strength in the direction of the principal axis of the material. The cross-sectional area of the first and second portions of the segment at the first and second ends of the joint region, yet outside of (i.e., not abutting) the fused portion, is greater than the cross-sectional area of the first and second portions of the joint region which abut the fused portion.
In one embodiment, the cross-sectional area of the first and second portions of the segment at the first and second ends of the joint region, yet outside of the fused portion, is approximately equal to the cross-sectional area of a segment of the elongated material outside of the joint region.
In a preferred embodiment, the total cross-sectional area of the first and second portions of the joint region which abut the fused portion is a minimum at approximately the midpoint of the fused portion. In a more preferred embodiment, the total cross-sectional area of the first and second portions of the segment at the midpoint of the fused portion is approximately half the total cross-sectional area of the first and second portions at the first and second ends of the joint region and outside of, or not abutting, the fused portion. In an especially preferred embodiment, the change in cross-sectional area of the first and second portions of the segment, per unit length of those portions, is substantially constant over the length of the fused portion of the joint region.
The elongated material may comprise a substantially monofilamentous material, such as, for example, a polymer. In a preferred embodiment, the elongated material is a thermoplastic polymer, such as a surgical suture material.
The segments of elongated material are preferably joined in a weld at the joint region. The weld can be effected with various types of energy, such as, for example, ultrasonic, laser, electrical arc discharge, and thermal energy.
The loop of elongated material can be made by joining portions of a single segment of the elongated material. Alternatively, the loop can be made by joining portions of multiple segments of the material.
The elongated material itself can comprise a single strand of a substantially monofilamentous material. Alternatively, the elongated material can comprise multiple strands of a substantially monofilamentous material which can be twisted, braided or otherwise interlinked.
Upon application of a tensile force to the joint region in the direction of the principal axis of the material, the first and second portions of the joint region are loaded substantially in tension, and the fused portion of the joint region is loaded substantially in shear. In a preferred embodiment, the following equation,
Awxcfx84fwxe2x89xa7Au"sgr"fu,
is preferably substantially satisfied. Aw is the shear area of the fused portion, xcfx84fw is the shear stress to failure of the fused portion, Au is the total cross-sectional area of the first and second portions near the first and second ends of the joint region and outside of (not abutting) the fused portion, and "sgr"fu is the tensile stress to failure of the first and second portions near the first and second ends and outside of (not abutting) the fused portion.
According to another aspect of the invention, there is provided an ultrasonic welding apparatus which includes a first member having a first suture-contacting surface, a second member having a second suture-contacting surface, and means for moving the first member relative to the second member to define a gap between the respective suture-contacting surfaces. The first member is capable of vibrating and delivering mechanical energy at ultrasonic frequencies. The second member is stationary relative to the first member. A fixture element is adapted to receive and maintain two or more segments of a material to be welded in a predetermined alignment in the gap between the first and second surfaces of the first and second members during a welding operation. The contour of at least the first surface substantially corresponds to the contour of a segment of the material to be welded so as to promote acoustic coupling therebetween and establish substantially continuous contact between the first surface and the segment over the length of the first surface.
In one embodiment, one of the first and second surfaces is substantially convex and the other of the surfaces is substantially concave. In another embodiment, one of the first and second surfaces is substantially convex or substantially concave, and the other of the surfaces is substantially flat. In yet another embodiment, both of the first and second surfaces are substantially convex. In still another embodiment, both of the surfaces are substantially flat.
The radius of curvature of the convex suture-contacting surface is preferably not greater than the radius of curvature of the concave suture-contacting surface. In the case in which both the first and second members have convex suture-contacting surfaces, the respective radii of curvature of the convex surfaces can be different, or they can be substantially identical.
In another embodiment, the second member comprises a plurality of coupling portions which couple together to form the second surface during a welding process and separate after completion of the welding process to release the loop.
According to another aspect of the invention, an ultrasonic welding apparatus as described above includes first and second members with patterned first and second suture-contacting surfaces. The patterned surfaces can be complementary or non-complementary, and the surface patterns on each member may vary in either a periodic or an aperiodic manner.
These and other features of the invention will be more fully appreciated with reference to the following detailed description which is to be read in conjunction with the attached drawings.